Local actions and action on the system

Simple statistical process control techniques can detect special causes of variation.Discovering a special cause of variation and taking proper action is usually the responsibility of someone who is directly connected with the operation.Although management must sometimes be involved to correct the condition,the resolution of special cause of variation usually requires Local Action .This is especially true during the early process improvement efforts.As one succeeds in taking the proper action on special causes,those that remain will often require management action,rather than local action.

These same simple statistical techniques can also indicate the extent of common causes of variation,but the causes themselves need more detailed analysis to isolate.
The correction of these common causes of variation is usually the responsibility of management.Sometimes people directly connected with the operation will be in a better position to identify them and pass them on to management for action.Overall,though,the resolution of common causes of variation usually requires action on the system.

Only a relatively small proportion of excessive process variation-industrial experience suggests about 15%-is correctable locally by people directly connected with the operation.The majority-the other 85%-is correctable only by management action on the system.

Confusion about the type of action to take is very costly to the organization,in terms of wasted effort,delayed resolution of trouble,and aggravated problems.It may be wrong,for example,to take local action (e.g., adjusting a machine)when management action on the system is required (e.g., selecting suppliers that provide consistent input materials).

Variation : Common and Special Causes

No two products or characteristics are exactly alike,because any process contains many sources of variability.The differences among products may be larger,or they may be immeasurably small,but they are always present.The diameter of a machined shaft,for instance,would be susceptible to potential variation from the machine (clearance, bearing wear),tool (strength, rate of wear),material(diameter,hardness),operator(part feed,accuracy of centering),maintenance (lubrication, replacement of worn parts),and environment (temperature,constancy of power supply).

Some sources of variation in the process cause short-term,piece-to-piece differences eg.,backlash and clearance within a machine and its fixturing, or the accuracy of a bookkeeper's work.
Other sources of variation tend to cause changes in the output only over a longer period of time,either gradually as with tool or machine wear,step-wise as with procedural changes,or irregularly,as with environmental changes such as power surges.Therefore,the time period and conditions over which measurements are made will affect the amount of the total variation that will be present.

The distribution can be characterized by

1-Location
2- Spread (span of values from smallest to largest)
3-Shape (the pattern of variation-whether it is symmetrical,skewed,etc.)

Common causes refer to the many sources of variation within a process that has a stable and repeatable distribution over time.This is called "in a state of statistical control".Common causes behave like a stable system of chance causes.If only common causes of variation are present and do not change,the output of a process is predictable.

Special causes (often called assignable causes) refer to any factors causing variation that are not always acting on the process.That is,when they occur,they make the (overall) process distribution change.Unless all the special causes of variation are identified and acted upon,they will continue to affect the process output in unpredictable ways.If special causes of variation are present,the process output is not stable over time.